What do you mean by 'not working'? Do you get an error? Or an incorrect plot? And why do you need to save to .txt or .xls to plot? Your code shows nothing to do with saving to a different file format, which supports the fact that it is not a necessary part of plotting data.
So is your question about plotting or saving data to .txt or .xls?
On a side note, you should presize 'intensity' as e.g.
intensity = zeros( NPoints, 1 );
if you wish to assign to it in a loop like that as it is more efficient than growing it in a loop. M-Lint should highlight this with an orange underline that you should always take note of in your code.
